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Can a cat throw misfire codes?

2501 Views 17 Replies 5 Participants Last post by  euro_flaw
97 E420 142000. It happened all of a sudden, stopped at 7-11, started the car, running very rough. Restarted, no help. Felt like misfiring 2 cylinders on the same bank. I had to fly cross country for a month to help my parents the next day, so I parked it. While I was gone my gurl drove it "sparingly" presumably. When I get back the problem is still the same, codes say cyl 1 misfire, cyl 3 misfire, and random cyl misfire. Plugs are about 2 years old so I start with that. Every plug looks like it's firing even, no carbon, but well used. The only issue I could see was #3 was loose enough I didn't need a ratchet to loosen it and there was approx 1/2" of oil in the plug well, but the plug itself was not showing any signs of fouling or oil in chamber. They were all Bosch super +, no signs of anything serious.
When I fire it up it runs much better, just a slight, random miss that smooths out with rpm's, more like a small vacuum leak. Leaving the top cover off so it's free breathing, by the time I get to the end of my block the "check engine electronics" reappears, and it stumbles at the stop sign. Thinking of the 1/2" of oil in #3 plug well, I decide to "clean it out". Huge difference in power. It never had this kind of power before, I'm thinking that might be that. But when I get back it still has that slight stumble at idle that goes away quickly with rpm, and when I look underneath the front passenger catalytic converter is about to go supernova it's so hot. The other 3 are hot, but not glowing so bright they look look like they might melt. The codes say cyl 1, 3, and random misfire only, no cat codes.
So here I am, new day. I'm next going to swap 1&3 coils to 5&7 to see if the misfire follows, and run it with the the top cover on, then off, to see if that has any effect on power. I'm thinking (praying actually) that the misfires are sending unburnt gasses to the cats, and the passenger cat by some twist of physics is receiving more gas than the other making it hotter, and that by taking off the top cover I get so much unrestricted air it irrefutably proves alll I need is a cold air intake to run laps as fast as nascar.
Any thoughts? Can it be a bad cat? Can a plugged cat throw misfire codes?
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I personally changed all vacuum lines I could find last year which fixed a slew of problems. How ever, I have yet to find any resource that shows ALL the vacuum lines and their respective locations. The gurl just had to go shopping so I haven't had a chance to try anything yet, ( poor decision, I know) , so I've been continuing the rebuild on my sons mustang until she gets back. Is there a resource that definitively shows ALL the vacuum lines? I'm kinda under the gun as the registration runs out on 3/04/15, and the state simply don't care I've been off for a month after shelling out the $ for a cross country trip and all the incidentals around my folks place. ( do you have any idea how much a rear tire costs for a Massey ferguson tractor?) I really appreciate the input, my friends. This is by far the most important resource available to all of us. Shared collective knowledge. Too bad the potential to exploit that is so high.
back pressure on both cats = .5 lb at idle- 1 lb at 2000 rpm steady and quick snap to 4500. I honestly thought it was the cats and nearly shelled out big $ for new. I highly recommend this test before replacing cats for any reason, either done yourself or a disinterested mechanic. (not trying to $$ for cats)

vacuum test = 13 at idle, dropped to nothing then back to 14 at 2000 rpm. removed a vacuum line and noticable difference vacuum and performance.

while engine was running unplugged the coils from cylinders 1 & 3 = no change at all. unplugging any other coil resulted in engine running worse. BINGO! at the very least a large part of the problem.

Heres where I got extreemely lucky. Since i had to go to the salvage yard anyway for a wiper motor for my truck, I purveyed the selection of benz's in the yard and spotted a relativly fresh 420. Along with a few other little do-dads I found that 6 of the 8 coils had been replaced recently. They looked brand new. I took 'em all. $5.50 each for Gods sake, even the 2 that looked older were in better shape than mine. when i got home i replaced all 8 with the ones i just got and 'Voila, smooth as silk. i could barely feel the engine running with my hand on the valve cover. An exhilerating test drive and i check the cats. Neither are even slighty red. I check the vacuum and now its steady 18 at idle, drops off at 2000 and comes back up to 18. It runs like it was a new car. Passed smog with no prob, now im good to go.

I really appreciate all the input from every one. It really makes for a thorough trouble shoot with so many suggestions and things I never would have considered otherwise. The back pressure test for the cats needs to be an absolute must suggestion for anyone thinking about their cats. That certainly saved me some coin. Thanks again.
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Thanks again. Be looking for more questions as im looking into the suspension. It just seems to me like it should ride better.
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